With a light source having a wavelength of a band of 1.55 .mu.m which is intended to provide a form of light communication over long distances with a large capacity, it is required that it has a single longitudinal mode which is stable, that it can be monolithically integrated with other functional devices with ease, and so on.
A dynamic single mode laser is a suitable form of laser which can satisfy these conditions.
Dynamic single mode laser is include distributed feedback type lasers (DFB), distributed Bragg reflector type lasers (DBR), compound resonator lasers, and so on.
Among these, the distributed Bragg reflector type laser is of particular interest.
In a distributed Bragg reflector type laser, a diffraction grating is formed on a waveguide and used as a distributed Bragg reflector.
This kind of laser has many advantages such as the fact that the stable single longitudinal mode operation can be easily maintained upon high speed modulation, that the laser can be monolithically integrated with other functional devices with case, that a length of a laser resonator can be set to a short cavity, and so forth.
Distributed Bragg reflector type lasers are mainly classified as DBR-ITG type lasers which have an integrated twin-waveguide structure and DBR-BJB type lasers which have a direct coupling structure.
The first of these type is the Distributed Bragg reflector integral twin-guide type laser which has two waveguides. An external waveguide is extended to the outside. An active waveguide is formed over that waveguide through a buffer layer. The distributed Bragg reflector surface is provided on the external waveguide.
This kind of laser involves the problem that part of the light is reflected between the active waveguide and the external waveguide. That is, there is a problem since the active waveguide is formed over the external waveguide and the coupling state therebetween is therefore inferior and the reflection loss in the coupling portion is large.
In a DBR-BJB type laser, both the active waveguide and the external waveguide are arranged in an almost rectilinear configuration. Namely, neither of the waveguides exist in the vertical direction but are arranged in the same plane; thus the reflection loss in the coupling portion is small.
In actual practice, however, a level difference can easily occur at the boundary between the two waveguides. There are certain drawbacks in that the active waveguide and external waveguide are incompletely coupled in the level difference portion and the light is reflected by this level difference portion.
Due to this reflection loss, the coupling efficiency between these waveguides is low. Consequently, the conventional DBR type lasers experience problems in that multi-mode oscillation can easily occur and the differential quantum efficiency is low.
With a view to solving these problems, Suematsu, Arai, Tohmori, et al invented the distributed Bragg reflector type (BIG-DBR type) semiconductor laser, their patent application therefor (Japanese Patent Application No. 60-12181, JP-A-61-171190) having been filed on Jan. 25, 1985, and laid open on Aug. 1, 1986.
The laser has a waveguide structure wherein an active waveguide is surrounded by an external waveguide.
In the central portion of this structure, the external waveguide is provided over the active waveguide (which is opposite to the case of the ITG type laser), while in the side portions, the external waveguide is disposed over an extended surface of the active waveguide. Thus the active waveguide and external waveguide are coupled in two directions, i.e., in the vertical direction and in the direction of the horizontal surface, whereby the coupling efficiency is improved. In actual practice, however, there is a serious problem with respect to the yields obtainable with this laser.
This point will now be explained with reference to FIGS. 8 to 11, which are cross-sectional views showing the manufacturing steps.
An InGaAsP active waveguide 12 and an InP depression layer 13 are sequentially formed on an p-type InP (100) substrate 11 by a liquid phase epitaxial method.
Next, the InP depression layer 13 and InGaAsP active waveguide layer 12 are etched with the central portions retained. At this time, a part of the upper surface of the InP substrate 11 is also etched.
A grating 16 is formed on the upper surface of the exposed InP substrate 11. FIG. 8 is a cross-sectional view showing the state of a laser formed by the foregoing steps.
As will be understood from FIG. 8, an edge surface U of the InP depression layer 13 and an edge surface W of the InGaAsP active waveguide 12 form one flush surface. This surface will be parallel to or slightly inclined from a (110) cleavage plane.
An InGaAsP external waveguide layer 14 is then formed on the depression layer 13 by the liquid phase epitaxial method.
It is desirable that the external waveguide layer 14 is so formed as to uniformly cover the upper surfaces of both the substrate 11 and the depression layer 13.
However, there is a problem in that the InGaAsP external waveguide layer 14 is not properly formed in the immediate vicinity of the InGaAsP active waveguide layer 12 and InP depression layer 13.
FIG. 9 shows such an improper state.
The external waveguide layer 14 grows in an epitaxial manner on the InP substrate 11 and also on the depression layer 13.
However, in the regions near the edge surfaces U of the depression layer 13 and the edge surfaces W of the active waveguide 12, the layer 14 hardly grows at all and gap portions G remain.
FIG. 10 is a cross-sectional view showing a state similar to FIG. 9. The edge surfaces U and W of the depression layer and active waveguide are slightly inclined in the direction toward the (111) plane. Actually, although it is unclear whether the edge surfaces U and W are inclined in the direction of the (110) plane or the (111) plane, in either case the external waveguide layer fails to grow in a state in which it intimately connects with the edge surfaces W and U.
FIG. 11 shows a special example of FIG. 9. It will be understood from this diagram that the InGaAsP external waveguide layer 14 is continuously formed without any gap at the edge surfaces U and W, but even in this continuous state dimples 18 remain.
In the case of the example shown in FIG. 11 the InGaAsP external waveguide layer 14 is continuously formed on the InP depression layer 13 and InP substrate 11, and the active waveguide and external waveguide are continuous with the edge surfaces W. However, although both the waveguides are coupled and the light in the active waveguide can be transmitted to the external waveguide, it is difficult to obtain a proper coupling state between them due to the influence of the dimples.
Lasers with structures of the described above have in fact been manufactured and, as shown in FIGS. 9 and 10, the external waveguide is interrupted on both sides of the active waveguide with gaps remaining. Almost all of the manufactured lasers of this type are like this. Since the coupling efficiency is 0, these products cannot function as lasers. There have been cases where lasers of the type shown in FIG. 11 have been produced at a low yield. However, the yield is only a few % and it is difficult to manufacture products which can be used as lasers in practice.
The reason why the external waveguide is cut each side of the active waveguide and depression layer is unclear. A possible reason is that the edge surfaces U of the depression layer and the edge surfaces W of the active waveguide are in the (111) mesa directional plane. There is a possibility that such edge surfaces are oblique surfaces as shown in FIG. 10.
It is a known fact that in liquid phase epitaxy crystal growth hardly ever occurs in the (111) mesa directional plane of InP.